proto: introduce aeon_internal.proto

This patch adds `aeon_internal.proto` which will be used to implement
the internal RPC.

Author: ImeevMA

aeon_internal.proto

@@ -0,0 +1,15 @@
+syntax = "proto3";
+
+package aeon;
+
+// Internal API to Aeon - a distributed database based on Tarantool.
+service InternalService {
+  // Pings the server.
+  rpc InternalPing(InternalPingRequest) returns (InternalPingResponse) {}
+}
+
+// Pings the server.
+
+message InternalPingRequest {}
+
+message InternalPingResponse {}